In Mass Effect 1 you find Matriarch Benezia on Noveria, where she is working on a project to breed Rachni, which were meant to be part of Saren's army to help him rule the galaxy. She was found by Shepard beside the Rachni Queen and attacked Shepard and his squad members. She was able to break free of Saren's will for a moment to warn them about Saren's power and motives, but was unable to muster enough mental strength to continue and finally succumbed to his will again causing Shepard to kill her. Liara was not sad of the death of her mother because she wanted to remember her as the person before Saren took control of her.